The lucerne flea, Sminthurus viridis (Collembola: Sminthuridae), is a springtail that is found in both the northern and southern hemispheres but is restricted to areas that have a Mediterranean-type climate. It is thought to have been introduced to Australia from Europe and has since become a significant agricultural pest of crops and pastures across the southern states. It is not related to the fleas which attack animals and humans.
The adult lucerne flea is approximately 3 mm long with light green-yellow colouring and an irregular pattern of darker patches over the body. Lucerne fleas are wingless, have globular abdomens and can jump large distances relative to their size. Their mottled colouration, small size and elusive habits can often make detection difficult.
Eggs, which are laid in batches, are covered in a soil layer making them almost impossible to detect in the field. The eggs are yellow-cream and about 0.3 mm in diameter. The newly hatched nymphs are approximately 0.75 mm long and are pale yellow in colour. Young nymphs resemble adults except they are much smaller in size and will moult several times before reaching maturity.
Lucerne fleas have a characteristic ability to 'spring' off vegetation when disturbed. This is due to a stiff appendage folded under their abdomen called a furcula, which is unfolded with such speed and force that it launches the lucerne flea into the air.
|Figure 2: The known distribution of lucerne flea in Australia|
Genetic analysis of populations suggests lucerne fleas have only limited movement between regions. It is likely that both adults and eggs are spread to new areas through the transport of hay and soil. Movement may also occur during summer from over-summering eggs being transported by wind.
While regional movement of lucerne flea is limited, anecdotal evidence suggests that the incidence of lucerne flea is increasing, even in areas of continuous cropping. The increased adoption of minimum and no-tillage may be contributing to the pest's increased survival and range expansion.
The lucerne flea has a similar seasonal biology to other important pests of establishing crops, such as the redlegged earth mite. Lucerne fleas generally hatch from over-summering eggs in March-April following soaking autumn rains. They reproduce sexually and, depending on favourable temperatures and moisture availability, may go through as many as three to five generations between autumn and spring. Each generation takes three to five weeks, with each female capable of laying up to three batches of eggs during this time.
Winter eggs are laid in moist crevices on the soil surface in batches of about 20-60, usually under vegetation and debris. Females then excrete a fluid substance containing ingested soil and glandular secretions over the eggs. This acts to both camouflage and protect the eggs.
At the onset of warmer and drier conditions, over-summering eggs are produced which are protected from desiccation by a clay cement layer excreted by females. Consequently, lucerne fleas are more common on heavier loam/clay soils and are rarely found on sandy soils. The protective coating also prevents eggs from hatching when rain is insufficient for lucerne flea development or for the establishment of host plants.
Behaviour and damage
|Figure 3: Lucerne fleas attacking clover. Photo: A. Weeks (CESAR).|
Both nymphs and lucerne flea adults are pests of broad-leaved plants such as clovers, lucerne and capeweed, and can also cause considerable damage to canola, field peas, lupins and faba beans. Although grasses and cereals are non-preferred hosts, lucerne flea can also cause damage to ryegrass, wheat and barley crops. In pastures, lucerne fleas have a preference for subterranean clover and lucerne.
Lucerne fleas move up plants from ground level, eating tissue from the underside of foliage. They consume the succulent green cells of leaves through a rasping process, avoiding the more fibrous veins and leaving behind a layer of leaf membrane. This makes the characteristic small, clean holes in leaves which can appear as numerous small 'windows'. In severe infestations this damage can stunt or kill plant seedlings.
|Figure 4: Typical lucerne flea feeding damage to clover|
Monitoring is the key to reducing the impact of lucerne flea. Crops and pastures grown in areas where lucerne flea has previously been a problem should be regularly monitored for damage from autumn through to spring. Susceptible crops and pastures should also be carefully inspected for the presence of lucerne fleas and evidence of damage.
It is important to frequently inspect winter crops, particularly canola and pulses, in the first three to five weeks after sowing. Crops are most susceptible to damage immediately following seedling emergence. Pastures should be monitored at least fortnightly from autumn to spring, with weekly monitoring preferred where there have been problems in previous years.
Lucerne fleas are often concentrated in localised patches or 'hot spots' so it is important to have a good spread of monitoring sites within each paddock. Examine foliage for characteristic lucerne flea damage and check the soil surface where insects may be sheltering.
Some sprays require application at a particular growth stage, so it is also important to note the growth stage of the population. Spraying immature lucerne fleas before they have a chance to reproduce can effectively reduce the size of subsequent generations.
Lucerne fleas compete for food and resources with other agricultural pests such as redlegged earth mites and blue oat mites. This means control strategies that only target one species may not reduce the overall pest pressure because other pests can fill the gap. It is therefore important to assess the complex of pests before deciding on the most appropriate control strategy.
Lucerne fleas are commonly controlled post-emergence, usually after damage is first detected. Control is generally achieved with an organophosphate insecticide (eg. omethoate). In areas where damage is likely, a border spray may be sufficient to stop invasion from neighbouring pastures or crops. In many cases, spot spraying, rather than blanket spraying, may be all that is required.
If the damage warrants control, treat the infested area with a registered chemical approximately three weeks after lucerne fleas have been observed on a newly emerged crop. This will allow for the further hatch of over-summering eggs but will be before the lucerne fleas reach maturity and begin to lay winter eggs.
In pastures, a follow-up spray may be needed roughly four weeks after the first spray to control subsequent hatches, and to kill new insects before they lay more eggs. Grazing the pasture before spraying will help open up the canopy to ensure adequate spray coverage. The second spray is unlikely to be needed if few lucerne fleas are observed at that time.
Crops are most likely to suffer damage where they follow a weedy crop or a pasture in which lucerne flea has not been controlled. As such, lucerne flea control in the season prior to the sowing of susceptible crops is recommended.
Caution is advised when selecting an insecticide. Several chemicals registered for redlegged earth mites (i.e. synthetic pyrethroids such as cypermethrin) are known to be ineffective against lucerne flea. When both lucerne fleas and redlegged earth mites are present, it is recommended that control strategies consider both pests, and a product registered for both is used at the highest directed rate between the two to ensure effective control.
Information on the registration status, rates of application and warnings related to withholding periods, OH&S, residues and off-target effects should be obtained before making decisions on which insecticide to use. This information is available from the DPI Chemical Standards Branch, chemical resellers, APVMA and the pesticide manufacturer. Always consult the label and MSDS before using any insecticide.
Biological and cultural control
Several predatory mites, various ground beetles and spiders prey upon lucerne fleas. Snout mites (which have orange bodies and legs) are particularly effective predators of this pest. The pasture snout mite (Bdellodes lapidaria) and the spiny snout mite (Neomulgus capillatus), have been the main focus of biological control efforts against lucerne flea.
The pasture snout mite was originally found in Western Australia but has since been distributed to eastern Australia and there are some examples of this mite successfully reducing lucerne flea numbers. Although rarer, the spiny snout mite can also drastically reduce lucerne flea populations, particularly in autumn.
|Figure 5: Predatory adult snout mite. Photo: A. Weeks (CESAR).|
Appropriate grazing management can reduce lucerne flea populations to below damaging thresholds. This may be because shorter pasture lowers the relative humidity, which increases insect mortality and limits food resources.
Broad-leaved weeds can provide alternative food sources, particularly for juvenile stages. Clean fallowing and the control of weeds within crops and around pasture perimeters, especially of capeweed, can therefore help reduce lucerne flea numbers.
Other cultural techniques such as cultivation, trap and border crops, and mixed cropping can help reduce overall infestation levels to below economically damaging levels, particularly when employed in conjunction with other measures. Grasses are less favourable to the lucerne flea and as such can be useful for crop borders and pastures.
In pastures, avoid clover varieties that are more susceptible to lucerne flea damage and avoid planting susceptible crops such as canola and lucerne into paddocks with a history of lucerne flea damage.